Vapor-liquid contact trays are used in mass transfer or exchange columns to facilitate contact between, for example, upwardly flowing vapor streams and downwardly flowing liquid streams. The trays are conventionally horizontally disposed within the columns to provide a horizontal surface across which the liquid streams may flow. The trays are typically formed from a solid sheet-like material (e.g., a plurality of panels) and contain a plurality of apertures which allow vapor to flow upwardly through the tray for interaction with liquid flowing across the top surface of the tray. In trays known as sieve trays, the apertures are sized small enough so that during operation of the column the pressure of the vapor passing upwardly through the apertures restricts or prevents liquid from passing downwardly through the apertures. In other types of trays, movable valves or stationary structural elements such as bubble caps can be provided over the apertures to seal against the downward passage of liquid.
Downcomers are used in combination with the vapor-liquid contact trays described above to provide a passageway for liquid to pass downwardly from one tray to an underlying tray. In single pass tray arrangements, the downcomers are provided at opposite ends of vertically adjacent trays so the liquid flows completely across one tray from the inlet end to the outlet end before it enters the downcomer for passage to the next lower tray. The liquid then flows in the opposite direction across the lower tray (or in some instances trays that generate a circular flow are utilized such that liquid flows in the same direction on each tray) and enters the associated downcomer for passage to and across lower trays in the same back and forth fashion. In two-pass tray arrangements, the tray is split into two streams which travel in opposite directions on each tray. A center downcomer is provided on every other tray and two end downcomers are placed at opposite ends of intermediate trays to provide the double pass flow pattern. In addition to double pass tray arrangements, alternative multi-pass tray downcomer designs include three and four pass trays. A four pass tray has side and center downcomers on every other tray and has two intermediate or off-center downcomers on the other-alternating trays. The center and intermediate (or off-center) downcomers have liquid flowing from either side. These tray arrangements include chordal downcomer designs as they cut a chord across the tower. There also exists in the art multiple downcomer tray arrangements featuring downcomers that “hang” in the middle of the tray decks and can potentially receive liquid from the entire perimeter of the downcomer. These interior hanging downcomers come in various shapes and sizes as in rectangular or circular cross-sectioned downcomers.
A weir is also typically used at the outlet end of vapor-liquid contact trays to provide a mechanical seal on the upstream downcomer and also to cause liquid to accumulate on the top surface of the tray for enhanced interaction with the vapor bubbling upwardly through the apertures in the tray deck before entering the downcomer. The area of the tray deck which contains the apertures in vapor-liquid contact trays in referred to as the “active area” of the tray because the vapor-liquid interaction occurs above the tray in this area. The active area typically does not include the area at the inlet end of the tray deck which lies immediately below the outlet of the downcomer which is associated with the overlying tray. This area of the tray below the downcomer outlet is referred to as the downcomer receiving area and is typically a solid plate which receives the vertically flowing discharge from the downcomer and redirects it horizontally to flow across the tray.
One problem associated with conventional trays of the type described above is the tendency for liquid to flow in a non-uniform manner across the tray. Because the width of a circular tray increases in the direction of liquid flow from the inlet end to the midpoint of the tray and then decreases from the midpoint to the outlet end, the liquid tends to preferentially flow along the center portion of the tray. This often results in decreased tray performance as liquid stagnates or forms non-uniform gradients along the lateral edges or other portions of the tray. Previous attempts to reduce liquid stagnation and non-uniform gradients have included the use of apertures which redirect vapor from a vertical to a horizontal flow path. The apertures thus cause liquid in the vicinity of the apertures to flow in the direction of the redirected vapor. These apertures are spaced about the tray and are generally concentrated in those areas where liquid needs to be redirected in order to avoid stagnation or gradients. An example of a slotted sleeve tray directed at avoiding the above noted flow maldistribution problems is found in U.S. Pat. No. 4,101,610, which is incorporated by reference.
There is also a desire to increase the flow through processing rate, while at the same time avoiding any degradation of the vapor-liquid exchange. A faster flow of fluid across the tray can potentially provide for increased flow through rates and more efficient use of a column, but can also lead to degradation with respect to vapor/liquid contact efficiency as well as potential flow through problems as in exchange downcomer flooding and backflow problems. For example, there is avoided providing the flowing liquid with too much fluid momentum induced by push valves, as the high momentum fluid would tend not to fall down evenly into the downcomer, and the increased horizontal momentum could lessen the effectiveness of the upstream portion of the downcomer. That is, the increased horizontal momentum would tend to cause the liquid flow to seal or cover the top cross-section of the downcomer and thus prevent disengaging vapor from escaping out of the downcomer without backing up liquid entering.
U.S. Pat. No. 5,975,504, which is incorporated by reference herein, describes high vapor and liquid capacity counterflow fractionation trays with a number of parallel downcomers separated by planar tray decking having bidirectional slotted cap valve geometries including raised trapezoids arranged in two groups which each face the nearest downcomer, or are arranged in lines parallel to the downcomer sidewalls such that trapezoids of neighboring rows face opposite directions. In an alternate embodiment these bidirectional slotted cap valves are only located in the regions of the decking sections located next to the downcomers.
PCT Application PCT/EP01/01814 describes a gas-liquid contact tray comprising a bubble area and a plurality of downcomers having a downcomer opening, which downcomer openings are spaced in the bubble area, such that, when in use, a liquid enters the downcomer opening from opposite sides of the downcomer opening, wherein at the downcomer opening a downcomer is provided with at least two flow directing plates, wherein each flow directing plate has an upper end which extends above the tray and is inclined towards the direction of the liquid flowing towards the downcomer opening. Reference is also made to PCT Application PCT/EP01/01806 which describes a gas-liquid contact tray comprising a bubble area and one or more downcomers provided at its upper end with a downcomer opening for receiving liquid, wherein the downcomer opening and downcomer is provided with a flow directing plate, and wherein the flow directing plate has an upper end which extends horizontally in the direction of the bubble area. PCT Applications '01814 and '01806 are incorporated herein by reference as well.
The PCT/EP01/01806 describes in its background section German patent publication 764103 which is noted therein as using flat and curved impingement plates in the downcomer opening to limit the froth height in the downcomer, which in turn is said to prevent downcomer back-up. The publication is further described as being directed at so-called cross-over trays configurations for use in so-called foaming gas-liquid systems. DE 764103 is incorporated herein by reference as well.
There is lacking, however, a tray apparatus which provides an advantageous combination of high flow momentum and redirecting of the enhanced momentum flow to provide fast (and yet uniform) flow of fluid through the tray apparatus.